list of the action hooks available for use in plugin development in Version 2.1 and above of WordPress.

widgets_init – Used to register sidebars. This is fired at ‘init’, with a priority of 1.

customize_register – Runs on every request, allowing developers to register new theme options and controls for use with the Theme Customization API

admin_menu – This action is used to add extra sub menus and menu options to the admin panel’s menu structure.

admin_init – It’s triggered before any other hook when a user access the admin area. This hook doesn’t provide any parameters, so it can only be used to callback a specified function.